1. Field of the Invention
The present invention relates to methods and apparatus for controlling the shape of etch profiles in semiconductor etching processes. More specifically, the invention relates to methods and apparatus for tailoring etch profiles using removable plugs that are tailored to particular etching processes.
2. Description of the Background
Manufacturers of semiconductor integrated circuits (ICs) are faced with intense competitive pressure to improve their products and the processes of fabricating them. A major business motivation for these manufacturers is the lowering of production costs through improvement of product throughput, quality and complexity. These improvements are in part a result of improving control over the etch rate as well as etch uniformity within a process. Because of this motivation for IC manufacturers, there is a need for a low cost way to tune processes to provide uniformly high-quality products. Fulfilling that need would provide a manufacturer of such equipment a competitive edge.
One goal is to improve tool performance at low cost. The company that can enhance tool performance without increasing tool cost is in a position to increase profit margins. Especially in cyclical industries such as the semiconductor capital equipment industry, increased profit margins can have a dramatic impact on market penetration.
To improve tool performance, several challenges are presented, such as how to increase uniformity of an etch profile, how to achieve a proper chemical balance of plasma, and how to spatially alter the chemistry of plasma above a wafer.
Concerning how to increase uniformity of an etch profile, in spite of significant advances most etch processes still induce an undesirably non-uniform etch profile 31, as illustrated in FIG. 1. Etch profile 31 illustrates how a conventional arrangement of a scavenging plate 70 mounted on an aluminum electrode 40 and located with respect to a wafer 30 causes etching to occur faster in the center of the wafer than at its periphery. It is desired to have an etch profile that is substantially flat (constant) throughout the entire surface of wafer 30, rather than declining toward the periphery as shown in FIG. 1. Non-symmetrical exhaust flow, temperature variations, non-uniform plasma chemistry, non-uniform ion density and/or non-uniform gas supply not only cause variations in the etch rate, but also adversely affect selectivity and device side wall profiles. Accordingly, there is still a need in the art to increase uniformity of etch profiles.
Concerning how to achieve a proper chemical balance of plasma, it is recognized that plasma reactors exploit the formation of chemically active plasma using carefully selected gases. As integrated circuit manufacturers push tool vendors to achieve higher etch rates and tighter-controlled etching, the need for balancing the chemical composition of the plasma increases. A high etch rate demands a large volume of plasma at a high density. This is typically done using inductively coupled plasma. For fluorocarbon chemistries in oxide (e.g., SiO2) etch, an inductively coupled plasma reactor is highly dissociative, which, if left uncontrolled, results in a large number of fluorine radicals created within the plasma and a poor selectivity between oxide etch to silicon etch. Moreover, fluorine radicals degrade etch performance characteristics including side wall profiles, and are thus undesirable. Accordingly, there is still a need in the art to achieve a proper chemical balance of plasma.
Concerning the problem of how to spatially alter the chemistry of plasma above a wafer, the undesirably etch profile illustrated in FIG. 1 is well known. Numerous approaches to altering plasma chemistry have been considered, with varying degrees of success. One way to adjust the chemistry of plasma is to expose the plasma to a chemically active substance. In many applications, silicon reacts favorably with plasma, for example if there is atomic fluorine present. Consequently, one solution is to include a silicon plate in the upper electrode to act as a scavenger. However, the limitation of this approach is that scavenging occurs uniformly across the scavenging plate. One solution to this limitation is to induce a counter shape in the scavenging plate. However, the results of this approach are hard to control, and can be very expensive because the shape and structure of the costly scavenging plate itself are involved. Accordingly, there is still a need in the art for a way to spatially alter the chemistry of plasma above a wafer in a controllable and cost-effective manner.
Various patents disclose methods dealing with non-uniform etching or a related problem of non-uniform implantation. U.S. Pat. No. 4,340,461 (Hendricks et al.), “Modified RIE chamber for uniform silicon etching,” discloses a plasma enhancing baffle plate employed in conjunction with the anode of an RIE system to attempt to provide uniform silicon etching. Various aperture arrangements may be employed to provide different patterns of neutral etching species generation. U.S. Pat. No. 4,392,932 (Harra), “Method for obtaining uniform etch by modulating bias on extension member around radio frequency etch table,” describes a method for etching a semiconductor wafer on an RF etch table that employs a succession of different biases on an extension member positioned adjacent the periphery of the table. The aggregation of etch profiles attempts to produce a more uniform overall etch profile. U.S. Pat. No. 5,552,017 (Jang et al.), “Method for improving the process uniformity in a reactor by asymmetrically adjusting the reactant gas flow,” discloses a method for improving etch rate uniformity and chemical vapor deposition uniformity using an asymmetric gas distribution system to increase the reactant gas flow over regions of the wafer where the etch rates are low. U.S. Pat. No. 5,897,712 (Hanawa et al.), “Plasma uniformity control for an inductive plasma source,” discloses reducing an RF induction field over areas of a wafer experiencing higher etch or deposition rates. U.S. Pat. No. 6,132,632 (Haney et al.), “Method and apparatus for achieving etch rate uniformity in a reactive ion etcher,” discloses use of a magnet array for locally controlling etch rates across a cathode to attempt to produce a uniform etch rate distribution in a reactive ion etcher. However, each of these methods, involving baffle plates and aperture arrangements, biases for an RF etch table, asymmetric gas distribution, asymmetric RF induction fields, or magnet arrays, are complex to design, build and operate. A simpler, more direct and more cost-effective approach is needed.
It is to meet the foregoing needs, among others, that the present invention is directed.